Gym equipment for muscle development takes various forms depending upon the muscles to be exercised. The trainee applies the force of his or her muscles to the equipment by a handle, a pedal, a lever or other device. Generally, the trainee's muscle force is transmitted through levers, cables and pulleys to weights or springs that provide counterforces to the trainee's muscle force.
In positive muscle training the trainee moves the handle, pedal or the like against a counterforce and repeats this movement many times. For building muscles the last two or three repetitions are executed in what is called the performance range. This is beyond the threshold where the trainee's performance capacity is already overstretched. In this range the exhausted muscle force no longer quite suffices to complete the movement. The trainee then depends upon the assistance of another person who must finely and sensitively give him a little help so that the movement does not stall. If, however, the aid is too great, this is detrimental. However, it is very difficult for a training partner to correctly regulate this assistance because the training partner is experiencing the after effect of his own strenuous exercise. Training on ordinary apparatus therefore frequently is inadequate for training success. The necessity of outside assistance further requires adaptation to the training program helper, and costs the time of both training program helper and trainee.
Training machines with processor-controlled "brake motors" are known, which offer the desired resistance to movement according to a predetermined characteristic curve and under program control. In the performance range beyond the threshold the person under training, by pressing a button, can reduce the the brake resistance by stages when he sees that his movement could be stalled. However, this hurts the trainee's ability to concentrate on executing the exercise. When the "dead point" is overcome, the original loading should become effective again, but since a switch would have to be actuated for that purpose, this resetting is often omitted.
Manually reducing the braking force also may risk reducing the resistance too much whereby the training results are impaired.
If the trainee overestimates his capacity the braking forces may be reduced too little, in which case the movement cannot be completed and again the training suffers.